In a typical resin transfer molding process, a fiber preform of the article to be manufactured is placed within a mold cavity defined by a steel mold, and a liquid resin is injected into the mold cavity. The resin is absorbed by the fiber preform, and then cured to form the article of manufacture. Typical fibers used to make fiber preforms include fiberglass, graphite, carbon and aromatic polyamide fibers such as are sold under the name Kevlar, and the fibers are often braided or woven into a sheet form. The resins are typically epoxy resins, although other types of resins, such as polyester resins, are also employed.
In FIG. 1, a typical prior art system 2 for resin transfer molding is illustrated, which includes a steel mold 4 defining a mold cavity 5, a pressure pot 6 coupled to an injection port of the mold 4, a collection pot 7 coupled through a resin vent to the mold cavity, and a vacuum pump 8 coupled through the collection pot 7 to the resin vent and mold cavity 5. A structural preform (not shown) is placed within the mold cavity 5, which is formed at least in part by woven or braided fibers. The pressure pot 6 is filled with resin and heated to mold temperature, and a vacuum is drawn on the mold cavity 5 by the vacuum pump 8. Pressurized air is then introduced into the top of the pressure pot 6, which forces heated resin into the line coupled between the pressure pot 6 and the injection port, and in turn into the mold cavity 5. As the pressurized, heated resin is introduced into the mold cavity 5, it is absorbed by the fiber preform.
At selected time intervals after introduction of the heated resin into the mold cavity 5, an operator opens the line coupled to the resin vent and bleeds resin from the mold cavity 5 into the collection pot 7. As illustrated in FIG. 1, the collection pot 7 includes a sight glass 9 to enable the operator to view the released resin. If there are air bubbles within the resin, then the operator should know based on experience and skill that the mold cavity is not yet full (and that the fiber preform is not fully saturated with resin). The operator then closes the resin vent, pressurizes the pressure pot 6 to a predetermined pressure again, and introduces more pressurized resin into the mold cavity 5. This inspection process is repeated at spaced intervals in time until the operator determines based on experience and skill that the mold cavity 5 is purged of air, and is filled so that the fiber preform has fully absorbed the resin.
Each resin bleeding and inspection step is relatively time consuming, primarily because after the operator bleeds resin into the collection pot 7, the pressure pot 6 must be pressurized again. This bleeding and inspection step is typically repeated at least three or four times for each article that is molded, making the resin transfer molding process time consuming and relatively expensive. In addition, resin is wasted each time the operator is required to bleed the resin into the collection pot 6 to inspect the resin. Because this occurs several times during the molding of each article, the volume of wasted resin and corresponding wasted costs can be substantial.
Another drawback of such prior systems is that they rely on operator judgment to determine when the mold cavity is filled with resin. This typically results in poor repeatability, and lower overall quality of the products being produced. If the operator prematurely terminates the resin transfer molding process, the molded part will typically have voids caused by an insufficient absorption of resin by the structural preform, which usually renders the part not usable. This is particularly the case in the aerospace industry, wherein voids in aircraft components, for example, cannot be tolerated.
It is an object of the present invention to overcome the drawbacks and disadvantages of such prior art apparatus and methods for resin transfer molding.